Rib blockage delineation in anatomically intelligent echocardiography

ABSTRACT

An interactive visual guidance tool for an imaging acquisition and display system and configured for user navigation with respect to a blockage of a field of view ( 216 ) detects, and spatially defines, the blockage. It also integrates, with the image for joint visualization, an indicium ( 244 ) that visually represents the definition. The indicium is moved dynamically according to movement, relative to the blockage, of the field of view. The indicium can be shaped like a line segment, or two indicia ( 244, 248 ) can be joined in a “V” shape to frame a region of non-blockage. The defining maybe based on determining whether ultrasound beams in respective directions are blocked. Included, for deriving the image, in some embodiments are imaging channels for receiving image data for which a metric of coherence, i.e., similarity among channel data, is computed. The determination for a direction is based on the metric for locations in that direction. One application is navigating an ultrasound probe between blocking ribs ( 208, 212 ) to achieve a standard cardiac view.

FIELD OF THE INVENTION

The present invention relates to tracking a spatial limit of visual blockage and, more particularly, to an interactive visual guidance tool for such tracking.

BACKGROUND OF THE INVENTION

Successful ultrasound scanning relies strongly on the training and experience of the user. Echocardiography scans are especially challenging as the heart is surrounded by ribs and lung tissue. Ultrasound can barely penetrate calcified ribs (typically encountered in the apical view) and lung tissue because of severe acoustic impedance mismatch between them and surrounding body tissue. In addition, ultrasound absorption in ribs is quite high compared to that of soft tissue. To avoid artifacts, the user has to place the probe in the right position, i.e., to find a good acoustic window, for imaging. Conventionally, this is done solely based on real-time ultrasound images displayed onscreen. Although experienced users are usually capable of recognizing image degradation, and of improving image quality accordingly by moving the probe to a better position, less experienced users might acquire compromised images because of inferior hand-eye coordination and less awareness of artifacts.

SUMMARY OF THE INVENTION

What is proposed herein below is directed to addressing one or more of the above concerns.

There exists a need for a tool that can guide, in real time, and train inexperienced or less experienced users to position the probe for optimal image quality.

To guide and train inexperienced or less experienced users to place the probe in the right position, it is proposed herein to help users with hand-eye coordination by providing a reversed “V” display superimposed on the images. Each of the two legs of the “V” indicates an edge of blockage such as by a rib, one leg per rib for instance. Each of the two legs of the “V” moves in real time according to movement of the field of view of the probe relative to the blockage. The field of view may move according to movement of the probe for example. The leg movement may be seen onscreen as respective real-time rotations of the legs so that they, while maintaining their “V” shape, continue to frame whatever is the current region of non-blockage in the ultrasound image currently onscreen. The field of view may be three-dimensional, in the case of a matrix probe. Or the field of view may be within an imaging plane, in the case of matrix probes or one-dimensional, i.e., 1D, probes. The imaging plane may be rotated either electronically or by manual manipulative control, while the interactive feedback appears onscreen. In particular, the rotation can be such as to rotate the imaging plane into and out of alignment with respective x-planes, the indicia moving dynamically according to the rotating. More than one ultrasound view, and respectively more than one indicium, e.g., “V” leg, or indicium pair may be viewable onscreen, concurrently for example. In particular, presented for viewing may be one image in one imaging plane and simultaneously another image in another, different imaging plane or x-plane, the indicia in both images moving dynamically with a change in field of view relative to the blockage. The indicium may alternatively be a coloring added to the screen in place of or complementing the V-leg graphic overlay, so that the coloring appears, for example, throughout the region of blockage seen onscreen.

In accordance with an aspect of the present invention, an interactive visual guidance tool is for a device that includes a display; an imaging probe; and a view rendering module configured for presenting, on the display, an image acquired via the probe. The tool is configured for user navigation with respect to a blockage of a field of view. The tool includes: a blockage module configured for detecting, and spatially defining, the blockage; and a demarcation tracking processor configured for integrating, with the image for joint visualization on the display, an indicium that visually represents the definition. The tool is configured for moving the indicium dynamically according to movement of the field of view relative to the blockage.

As a sub-aspect, the image includes an ultrasound image.

In a further sub-aspect, the tool includes, for deriving the image, imaging channels for receiving image data via the probe. For the data received on the channels and with focusing delays having been applied, a metric of similarity among the data is computed. The defining is based on the metric.

In a still, further sub-aspect, the metric is representative of a correlation between beamsummed data of different apertures, the channels being divided among the apertures.

As a yet further sub-aspect, the apertures are interleaving complementary apertures.

In a particular, still further sub-aspect, the channels are randomly divided among the apertures.

In another sub-aspect, whether ultrasound beams in respective directions are blocked is determined, the defining being based on a result of the determining.

In a sub-aspect of this, the tool has, for deriving the image, imaging channels for receiving image data via the probe. A metric of coherence of the data received on the plural channels is computed. The determination of blockage/non-blockage for a direction is based on the metric for locations in that direction.

In one further sub-aspect of this, determination, in a direction, is based on whether the metric indicates a predetermined level of coherence for at least a predetermined number of locations, in that direction, for which the metric was computed.

In a different sub-aspect, the field of view is V-shaped with the point of the V at said probe.

As a related sub-aspect the indicium is shaped like a line segment.

In one sub-aspect of this, there is integrating, with the image for joint visualization on the display, a second indicium that visually represents the definition. The tool is further configured for moving the second indicium dynamically according to movement of the field of view relative to the blockage. The indicia jointly are V-shaped.

In another sub-aspect, the indicium represents a spatial border of the blockage.

As an alternative sub-aspect, the integrating involves adding, to the image, color as the indicium.

As an additional sub-aspect, the image is shaped like a sector of a circle.

In yet another sub-aspect, the probe is a matrix probe, with the field of view being three-dimensional. With respect to the same probe position, another indicium is integrated with another image for joint visualization on the display, the two images being in different imaging planes. The tool is further configured for moving said another indicium dynamically according to movement of the field of view relative to blockage in the respective, different imaging plane.

As a sub-aspect of the above, said another image and said image are displayed concurrently on the display.

In a specific sub-aspect, the detecting, and the defining, are with respect to, as a cause of said blockage, a rib of a human or of an animal.

In a particular sub-aspect, a fraction of sampled locations within the field of view that have valid data is calculated. An indicator of the calculated fraction is shown on the display.

In one version of the tool, the field of view is within an imaging plane. The blockage is with respect to imaging in the imaging plane.

In a further sub-version, the tool is configured for rotating the imaging plane into and out of alignment with respective x-planes and for moving the indicium dynamically according to the rotating.

In a still further sub-version, the imaging plane is transverse to one or more ribs causing the blockage.

In one more sub-aspect, the field of view is a field of view of the probe.

Details of the novel, real-time, interactive visual guidance technology are set forth further below, with the aid of the following drawings, which are not drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an ultrasound technician interactive guidance apparatus in accordance with the present invention;

FIG. 2 is an illustration showing an example of imaging-blockage demarcation, and the directions of blockage, in accordance with the present invention;

FIG. 3 is flow chart, and formula list, relating to coherence-based spatial discrimination and view blockage boundary determination based on the discrimination, in accordance with the present invention;

FIG. 4 is a conceptual diagram illustrating random complementary transducer array apertures, and their use in tracking spatial limits of visual blockage;

FIG. 5 is a flow chart of overall operation of one implementation of the apparatus of FIG. 1, in accordance with the present invention;

FIG. 6A is a flow chart of an example of coherence map derivation, in accordance with the present invention;

FIG. 6B is a flow chart of an example of boundary line determination, in accordance with the present invention; and

FIG. 6C is a flow chart of an example of updating presentation on a display, in accordance with the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 depicts, by way of illustrative and non-limitative example, an ultrasound technician interactive guidance apparatus 100. The apparatus 100 includes a medical ultrasound device 104 and an interactive visual guidance tool 108. Included in the tool 108 are a blockage module 112, a demarcation tracking processor 116, an imaging data coherence processor 120 and display control circuitry 124. The medical ultrasound device 104 includes a display 128; a view rendering module 132; a system controller 136; memory 140; an imaging probe 144 having a transducer array 148, imaging channels 152 and beamformers 154, 156; and user controls 160. The blockage module 112 detects, and spatially defines, the blockage. The demarcation tracking processor 116 integrates, with an ultrasound image for joint visualization on the display 128, an indicium that visually represents the spatial definition by the blockage module 112. The imaging data coherence processor 120 correlates imaging data acquired via complementary apertures of the probe 144.

FIG. 2 shows a representation of a fan-shaped ultrasound image 204 or sonogram acquired via the transducer array 148 with the imaging probe 144 placed intercostally and against the skin 150 of a human being or animal. The imaging plane is transverse to the ribs 208, 212 which partially block a field of view 216 of the probe 144. As discussed herein above, from the image 204 alone, without added graphics, the blockage may not be easily discernable to an inexperienced clinician. The ribs 208, 212 and/or lungs (not shown) cause blockage of the imaging. The blockage is characterizable by regions 228, 232 of blockage on both sides of a region 236 of non-blockage. Imaging of a heart 240 is partially blocked by the rib 212 shown on the right side. An onscreen indicium 244 visually represents the spatially defined blockage, is shaped like a line segment extending to a curved portion of the fan-shaped, or sector-shaped, image 204, and is superimposed on the image for joint visualization. The field of view 216 may change, by probe movement for instance. During the change, the indicium 244 moves circumferentially on-screen in real time to remain indicative of imaging blockage, e.g., by the rib 212 shown on the right, of the current onscreen image 204. In particular, the demarcation tracking processor 116 estimates an angle of a left edge 246, of the rib 212 shown on the right, relative to the imaging aperture of the transducer array 148. A second indicium 248 also tracks blockage in real-time based on a right edge 250 of the rib 208 shown in FIG. 2 on the left side, the indicia jointly remaining “V” shaped with the point of the “V” at the probe 144. Hash marks 254 running along the outside of the two legs of the “V” indicate the region of blockage. The ribs 208, 212 shown in FIG. 2 would not appear onscreen; instead, their presence is tracked by means of the indicia 244, 248. The heart 240 shown in FIG. 2 could appear onscreen as a graphic generated to represent a segmentation based on the ultrasound imaging. Or the heart 240 may appear onscreen as merely part of the ultrasound image 204. Although the focus of discussion herein relates to rib-based blockage, blockage can also be due to lung tissue, and the tracking indicia 244, 248 will automatically frame regions 228, 232 of blockage.

Coherence of channel data is used to detect blockage. Each channel delivers its respective radiofrequency data magnitude associated with its respective fixed transducer element or patch of elements. As ultrasound echoes return, their incident pressures on the elements are sampled quickly and periodically. The samples are delayed with respect to each other according to the line-of-sight travel time geometry of the field point being evaluated. Here, “coherence” means similarity among data recorded by different channels of an array after applying the above-mentioned receiving focusing delays.

One gauge of coherence is a beamsummed-data-based coherence estimation method, such as the one described in U.S. Patent Publication No. 2009/0141957 to Yen et al., the entire disclosure of which is incorporated herein by reference.

The estimation method can be tailored to detecting rib and lung blockage, and is demonstrated below using the two beamformers 154, 156. Let s_(j)(r, θ) denote the (real-valued) channel data at depth r along the receive beam in the direction θ, that data received by the j-th channel after applying the focusing delay, and let C₁ and C₂ denote the set of channels used in the first and the second beamformer 154, 156, respectively. The output of the k-th (k=1, 2) beamformer is b_(k) (r, θ), the formula for which is shown in FIG. 3. When all the channel data s_(j)(r, θ) are identical across channels, b₁(r, θ) and b₂ (r, θ) will be highly correlated no matter how C₁ and C₂ are chosen. On the other hand, when the channel data are mainly contributed by scatterers in sidelobe regions, the correlation between b₁ and b₂ can drop significantly if C₁ and C₂ are properly chosen. C₁ and C₂ can be complementary, interleaving apertures. In short, it is possible to distinguish between on-axis signals and off-axis signals based on correlation between b₁ and b₂. The output of the correlator is the correlation coefficient ρ(r, θ) of b₁(r, θ) and b₂(r, θ) defined as listed in FIG. 3, where w is a real symmetric weighting function. ρ(r, θ) is then lowpass filtered to get a smoothed correlation map {circumflex over (ρ)}(r, θ) which is used for blockage detection. The correlation coefficient ρ(r, θ) serves a metric of similarity, or metric of coherence, among the data received on the imaging channels with focusing delays having been applied. It is representative of a correlation between beamsummed data of different apertures, with the imaging channels being divided among the apertures. The division may be random, as discussed further below.

A flow diagram for the algorithm is shown in FIG. 3. Sums of s_(j)(r, θ) are taken for C₁ (step S310) and for C₂ (step S320). They are correlated to calculate the correlation coefficient ρ(r, θ) (step S330) which is low-pass filtered (step S340) to produce the smoothed correlation coefficient map, or “correlation map” {circumflex over (ρ)}(r, θ) used for blockage detection (step S350). The indicia 244, 248, each serving as a spatial border between blockage and non-blockage, are then generated for presentation in the inverted “V” display (step 360).

In a specific example, the data is acquired at 32 MHz sampling rate in a pulse-inversion mode using a probe having 80 transducer elements. Each frame has 44 beams and the beam density is 0.4944 beam/degree. The center frequency is 1.3 and 2.6 MHz on transmit and on receive, respectively. C₁={20-22, 26-28, 32-34, 38-40, 44-46, 50-52, 56-58} and C₂={23-25, 29-31, 35-37, 41-43, 47-49, 53-55, 59-61}. The weighting function w used in the correlator is a 51 (axially or in the r direction) by 1 (laterally or in the θ direction) boxcar and the smoothing filter is a 501 by 3 boxcar.

Due to the periodic structure of the apertures, sensitivity of the correlation coefficient ρ to off-axis signals varies periodically with the direction of off-axis signals.

This periodicity can be alleviated by randomizing sub-aperture sizes while still keeping both apertures complementary. In other words, the channels are randomly divided among the apertures.

An example of random complementary apertures is C₁={21-22, 26-28, 30-31, 35, 37, 39, 41-45, 49, 51, 53, 57, 60-61} and C₂={20, 23-25, 29, 32-34, 36, 38, 40, 46-48, 50, 52, 54-56, 58-59}.

To verify whether a beam, and thus its direction, is blocked, a count is made of the number of points with a correlation coefficient ({circumflex over (ρ)}) higher than 0.55 between 72 and 180 mm in depth. If at least 400 points (at 32 MHz sampling rate) in a beam have high coherence, this beam is considered penetrating into tissue. Otherwise it is considered blocked by a rib.

The upper bound of the depth range is not critical. 72 mm, much larger than the depth of human ribs in general, can be chosen as the lower bound because high coherence or correlation coefficient values might be present in regions right below a rib due to multiple reflections (or reverberation) and such reflections tend to fade away with depth.

The apertures described do not include channels in both ends of the full aperture. Though apertures can be extended to include those channels, the number of blocked beams might be underestimated if large apertures are used. This is because the correlation coefficient of complementary aperture outputs could still be high if part of the large complementary apertures is not blocked.

FIG. 4 illustrates random complementary transducer array apertures, and their use in tracking spatial limits of visual blockage. An aperture 404 (C₁) is set for the first beamformer 154 and an aperture 408 (C₂) is set for the second beamformer 156. Imaging channels 410 are represented along the horizontal axis. Beamsummed output of the apertures 404, 408 is correlated to discern indicia 412, 416, 420, 424 superimposed on two side-by-side displayed sonograms 428, 432 representative of different imaging planes.

FIG. 5 demonstrates an exemplary procedure 500 for overall operation of one implementation of the apparatus 100. The apparatus 100 is pre-set, during initialization, for interleaving complementary apertures, such as random interleaving complementary apertures (step S510). The correlation map of step S350 is produced (step S520). Based on the correlation map {circumflex over (ρ)}(r, θ), blockage boundary lines are set (step S530). The aperture settings are toggled back for acquiring an ultrasound image for display (step S540). The presentation is updated, as by refreshing the display, for movement of the field of view 216 (step S550). If the procedure 500 is to continue (step S560), the aperture setting are toggled back to yield (random) interleaving complementary apertures (step S570), and processing branches back to derivation of the correlation map in step S520.

FIG. 6A is a more detailed description of the correlation map formation in step S520. Imaging data is received by the channels 410 of the complementary apertures 404, 408 (step S604). Focusing delays are applied by channel. Each of the beamformers 154, 156 does this separately (step S608). The delayed data is beamsummed by the respective beamformers 154, 156; the beamsummed data of one of the beamformers 154, 156 is correlated with that of the other to calculate the correlation coefficient ρ(r, θ); the correlation coefficients ρ(r, θ) are ordered by imaging depth in the axial direction and laterally in the beam direction; and the coefficients are low-pass filtered and smoothed, to complete the map {circumflex over (ρ)}(r, θ), as in steps S310-S350 (step S612).

FIG. 6B is a more detailed description of boundary line determination of step S530. A current direction and a current imaging depth are respectively initialized to the first direction and the first depth. The depth, direction, and coherence counts are initialized to zero (step S616). If the current correlation coefficient ρ(r, θ) in the completed map exceeds the predetermined correlation threshold (S618), a predetermined level of coherence is deemed to exist for that location (r, θ) and the coherence count is incremented (step S620). In either event, if a next imaging depth exists (step S624), the depth count is incremented (step S628) and return is made to step S618. If, on the other hand, a next imaging depth does not exist (step S624), the depth count is cleared (step S630), and query is made as to whether the coherence count exceeds a predetermined qualification threshold, i.e., whether a sufficient number of correlation coefficients ρ(r, θ) for respective imaging depths along the current direction each indicate sufficiently high correlation (step S632). If the qualification threshold is exceeded (step S632), the direction is deemed to be qualified (step S636). In either case, if a next direction exists (step S640), the direction count is incremented (step S644), and return is made to step S618. Otherwise, if there is no next direction (step S640), the boundary lines are set at the leftmost and rightmost qualified directions to spatially define the blockage (step S648). The boundary lines in FIG. 2 are the indicia 244, 248.

As an alternative to line-by-line processing of the correlation map {circumflex over (ρ)}(r, θ), an image can be derived from the map by image or morphological processing such as dilation/erosion that rejects outliers and/or suppresses error. After processing, the indicia 244, 248 are the lines that frame the output region. They will inherently indicate onscreen, to the user, beam directions that lie outside the indicia, i.e., in which beams are blocked as presently evidenced by lack of imaging data coherence along the beam.

FIG. 6C is a more detailed description of updating or refreshing the onscreen presentation in step S550. Both the ultrasound image 204 and the indicia 244, 248 are updated for the current onscreen view (step S652). Since the just prior update, movement of the field of view 216, either by movement of the probe or rotation of the imaging plane, may have occurred. Therefore, the frame buffer is updated not only with the currently acquired ultrasound imaging for rendering an ultrasound image, but with graphics such as the indicia 244, 248 as overlays. The current onscreen view may accordingly have one or more fan-shaped images 204 with their respective indicia 244, 248. An ongoing indicator of the fraction of sampled locations within the field of view 216 that have valid data is also displayed onscreen (step S656), after having been refreshed to a current value. More specifically, the coherence count that has been incremented repeatedly in step S620 is divided by the total number of sampled locations (r, θ) to compute the fraction.

An alternative to the above-described coherence estimation is use of a coherence factor calculated by processing complex-valued channel data. The coherence factor is defined as

${{{CF}\left( {r,\theta} \right)} \equiv \frac{{{\sum_{j = 1}^{N}{s_{j}\left( {r,\theta} \right)}}}^{2}}{N{\sum_{j = 1}^{N}{{s_{j}\left( {r,\theta} \right)}}^{2}}}},$

where r is the depth along beam direction, θ is the beam direction, S_(j)(r, θ) is the complex-valued channel data at depth r received by the j-th channel after applying the focusing delay, and N is the number of channels. The term Σ_(j=1) ^(N)S_(j)(r, θ) (or |Σ_(j=1) ^(N)S_(j)(r, θ)|) in the numerator represents an image as a function of r and θ after coherent beamforming but before scan conversion and logarithmic compression. No more than a single beamformer is required. CF(r, θ) substitutes for the correlation coefficient ρ(r, θ) in the above-discussed mapping and indicium determination.

In the above-described embodiments, it is assumed that continuous ultrasound acquisition is accompanied with continuous update of the onscreen display. Alternatively, the onscreen display can be updated only when the field of view 216 changes. Thus, the updating can be responsive to probe movement detectable by an integrated electromagnetic (EM) sensor, as well as to image plane movement such as rotation. An example of such an EM sensor is seen in commonly-owned U.S. Pat. No. 7,933,007 to Stanton et al., the entire disclosure of which is incorporated herein by reference.

An interactive visual guidance tool for an imaging acquisition and display system and configured for user navigation with respect to a blockage of a field of view detects, and spatially defines, the blockage. It also integrates, with the image for joint visualization, an indicium that visually represents the definition. The indicium is moved dynamically according to movement, relative to the blockage, of the field of view. The indicium can be shaped like a line segment, or two indicia can be joined in a “V” shape to frame a region of non-blockage. The defining may be based on determining whether ultrasound beams in respective directions are blocked. Included, for deriving the image, in some embodiments are imaging channels for receiving image data for which a metric of coherence, i.e., similarity among channel data, is computed. The determination for a direction is based on the metric for locations in that direction. One application is navigating an ultrasound probe between blocking ribs to achieve a standard cardiac view.

In addition to making diagnostic cardiac examination performable by nurses or other clinicians who may be untrained specifically in sonography, the interactive visual guidance tool 108 can guide novice sonographers. The tool 108 can feature, for this purpose or this mode, a regular (grayscale) sonogram, along with the visual feedback described herein above. Alternatively, the novel visual feedback of the tool 108 can speed up the work flow of trained or experienced sonographers. The ultrasound technician interactive guidance apparatus 100, which includes the tool 108, may encompass a more comprehensive interactive visual guidance system such as that disclosed in commonly-assigned patent application entitled “Anatomically Intelligent Echocardiography for Point-of-Care” to Radulescu et al.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, instead of hash marks on the “V”, outwardly pointing arrows may be employed.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.

A computer program can be stored momentarily, temporarily or for a longer period of time on a suitable computer-readable medium, such as an optical storage medium or a solid-state medium. Such a medium is non-transitory only in the sense of not being a transitory, propagating signal, but includes other forms of computer-readable media such as register memory, processor cache, RAM and other volatile memory.

A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 

1. An interactive visual guidance tool for a device comprising a display; an imaging probe; and a view rendering module configured for presenting, on said display, an image acquired via said probe, said tool configured for user navigation with respect to a blockage of a field of view, said tool comprising: a blockage module configured for detecting, and spatially defining, said blockage; and a demarcation tracking processor configured for integrating, with said image for joint visualization on said display, an indicium that visually represents the definition, said tool configured for moving said indicium dynamically according to movement of said field of view relative to said blockage.
 2. The tool of claim 1, said image comprising an ultrasound image.
 3. The tool of claim 2, further comprising, for deriving said image, a plurality of imaging channels for receiving, via said probe, image data to which focusing delays are applied, said blockage module being further configured for computing a metric of similarity among the data received on the plural channels with said delays having been applied, said defining being based on said metric.
 4. The tool of claim 3, said metric being representative of a correlation between beamsummed data of different apertures, said plural channels being divided among said apertures.
 5. The tool of claim 4, said apertures being interleaving complementary apertures.
 6. The tool of claim 5, said channels being randomly divided among said apertures.
 7. The tool of claim 2, said blockage module being further configured for determining whether ultrasound beams in respective directions are blocked, said defining (S648) being based on a result of said determining.
 8. The tool of claim 7, further comprising, for deriving said image, a plurality of imaging channels for receiving, via said probe, image data, said blockage module being further configured for computing a metric of coherence of the data received on the plural channels, the determination for a direction from among said directions being based on said metric for locations in said direction.
 9. The tool of claim 8, the determination, in a direction from among said directions, being based on whether said metric indicates a predetermined level of coherence for at least a predetermined number of locations, in said direction, for which said metric was computed.
 10. The tool of claim 1, said field of view being V-shaped with the point of the V at said probe.
 11. The tool of claim 1, said indicium being shaped like a line segment.
 12. The tool of claim 11, said processor being further configured for integrating, with said image for joint visualization on said display, a second indicium that visually represents said definition, said tool being further configured for moving said second indicium dynamically according to movement of said field of view relative to said blockage, the indicia jointly being V-shaped.
 13. The tool of claim 11, said indicium representing a spatial border of said blockage.
 14. The tool of claim 1, said integrating comprising adding, to said image, color as said indicium.
 15. The tool of claim 1, said image (204) being shaped like a sector of a circle.
 16. The tool of claim 1, said probe being a matrix probe, said field of view being three-dimensional, said processor being further configured for, with respect to a same probe position, integrating, with another image for joint visualization on said display, another indicium, said another image representing a different imaging plane than that of said image, said tool being further configured for moving said another indicium dynamically according to movement of said field of view relative to blockage in said different imaging plane.
 17. The tool of claim 16, said another image and said image being displayed concurrently on said display.
 18. The tool of claim 1, said blockage module being further configured for said detecting, and said defining, with respect to, as a cause of said blockage, a rib of a human or of an animal.
 19. The tool of claim 1, configured for calculating a fraction of sampled locations within said field of view that have valid data and for showing, on said display, an indicator of the calculated fraction.
 20. The tool of claim 1, said field of view being within an imaging plane, said blockage being with respect to imaging in said imaging plane.
 21. The tool of claim 20, configured for rotating said imaging plane into and out of alignment with respective x-planes and for moving said indicium dynamically according to said rotating.
 22. The tool of claim 20, said imaging plane being transverse to one or more ribs causing said blockage.
 23. The tool of claim 1, said field of view being a field of view of said probe.
 24. A computer readable medium embodying a computer program for interactive visual guidance in connection with user navigation with respect to a blockage of a field of view, said field of view being acquired by an imaging probe movable under said guidance, said program having instructions executable by a processor for performing a plurality of acts, from among said plurality being the acts of: detecting, and spatially defining, said blockage; and integrating, with said image for joint visualization, an indicium that visually represents the definition and that dynamically moves according to movement of said field of view relative to said blockage. 